The present invention relates to voltmeters generally and to voltmeters for use in electrical power transmission line servicing and maintenance in particular.
Electricity transmitted through power lines destined for commercial, industrial and residential use can involve hundreds of thousands of volts and high currents. Inevitably, there is an element of danger in measuring the voltage on a transmission line because of the need to make contact with the line. Indeed, even the proximity to a high voltage line may be sufficient to cause a spark to jump through the air to the nearest object. Nonetheless, in installing, servicing and repairing power lines, there are various occasions when contact is made, such as when the voltage carried by a line must be measured.
Presently, high voltage phasing voltmeters use two test probes, which are each high voltage resistors housed in an insulated holder. The probes are electrically connected in series with a meter using a cable to connect them. The holders will have metal hooks or other fittings on their ends to facilitate a quick, good electrical contact with transmission lines. Often the meter is mounted to one of the two test probes and oriented so that the electric utility worker can read the voltage displayed on the meter. “Hot sticks” may be used to hold and elevate the entire assembly up to a transmission line. The meter may be designed to measure either voltage or current, but its display indicates voltage.
In addition to concerns for safety, there are a number of factors that introduce inaccuracies in these types of measurements. One of these factors is the existence of capacitive currents. However, the inaccuracies in phasing voltmeters attributable to capacitive currents are eliminated by the design disclosed and described in a commonly owned U.S. Pat. No. 6,459,252 issued Oct. 1, 2002, which is incorporated herein by reference.
Other factors that affect accuracy with phasing voltmeters have also been solved. On occasion, more than a few feet may separate power transmission lines. While the alternating current phasing voltmeter disclosed in the related U.S. Pat. No. 6,459,252, practically eliminates capacitive currents regardless of the length of the cable required to make measurements of power lines that are separated, it does not effectively address the problem of the physical problems in dealing with a long cable or the problem of having a cable that is not long enough. Commonly owned U.S. Pat. No. 6,617,840 issued Sep. 9, 2003 and U.S. Pat. No. 6,734,658, issued May 11, 2004, however, solve this problem to some extent by eliminating the cable altogether in favor of an alternating current phasing voltmeter in a master and slave probe combination that communicate voltage information wirelessly. The patent specification of U.S. Pat. No. 6,734,658 is also incorporated herein by reference. This phasing voltmeter also helps to overcome the natural reluctance of workers to apply test probes to power lines carrying very high voltages and it incorporates the accuracy improvements of U.S. Pat. No. 6,459,252.
However, there remains a need for a phasing voltmeter that is accurate, easy to read, and can be easily used when the transmission lines are separated by more than a few hundred feet, perhaps even separated by a distance on the order of a few miles.